Molten salt nuclear fission reactors are those where the fissile material is present in the form of a molten halide salt, usually chloride or fluoride. A novel design of such reactors was described in GB 2508537, in which the molten fuel salt was held in tubes surrounded by a second molten salt acting as a coolant. Control of reactivity of such reactors was proposed to be by using the negative temperature coefficient of reactivity to allow high temperatures to render the rector subcritical, by use of neutron absorbing control rods or by addition of the neutron absorbing material europium fluoride or cadmium fluoride to the coolant salt.
Both europium fluoride and cadmium fluoride have severe limitations for use as neutron poisons added to the coolant salt. Europium is a strongly electronegative metal which would make reduction of the fluoride to the metal, either electrolytically or chemically, impossible without also reducing less electronegative coolant salt components such as zirconium. Cadmium is relatively easy to reduce to the metal, as set out in GB 2508537, but the metal produced is highly volatile and toxic at the temperature of the coolant salt and would therefore require specialised handling which, in the context of a nuclear reactor, would be complex and expensive.
Europium and cadmium also have substantially lower absorption cross sections for fast neutrons than the boron more conventionally used as a neutron poison making them less useful.
Conventional water cooled and moderated reactors use sodium borate added to the water to reduce reactivity, with the advantage that the borate is easily removed from the water as needed. There would be great advantage to having an analogous method available for the molten salt reactor described in GB 2508537, however borate salts are not chemically compatible with the molten salt of the coolant. Use of boron as the control material is particularly valuable in fast spectrum reactors as the boron has a high neutron absorption cross section even in the fast neutron spectrum.